Method for optically recording information

ABSTRACT

A method for optically recording information on an optically re-writable information medium is described. A mark is formed on a recording medium by radiating a laser light having the strength of the short recording pulse signal followed by the off-pulse signal. The recording film is heated to form the mark and the mark is extended by thermal diffusion while the off-pulse signal is provided. Therefore, the length of the recorded mark ML is longer than the recording pulse width at a predetermined linear velocity. The erasing pulse signal follows the off-pulse signal and the extended recorded mark is erased by the erasing pulse signal. The recording method according to the present invention can form a mark on the recording medium by radiating the laser light having a shorter recording pulse width than a recording pulse width used in the conventional method to form a mark having the same length.

This application is a continuation of application Ser. No. 10/071,199,filed on Feb. 11, 2002 now U.S. Pat. No. 6,714,230, which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a method for opticallyrecording information and more particularly a method for opticallyrecording information in which a multi level signal is recorded on atrack.

2. Description of the Related Art

The Japanese Laid-Open Patent Application No.8-147695 (a prior artmethod {circle around (1)}) discloses such kind of technology. In theprior art method {circle around (1)}, although the power of a laserdiode is not decided in an ALPC zone, the laser diode is driven by asimple low bandwidth drive circuit with a multiple level drive current.First, the laser diode is driven on a trial basis in a Test Zone or aManufacturer Zone, which zones are placed at an inside and an outside ofan optical disc, and each zone has more than 100 tracks. In these zones,first, the laser diode is driven by the drive circuit having a currentlevel of Iw0. Next, the laser diode is driven by the drive circuithaving a current level of Iw1. Then, the laser diode is driven by thedrive circuit having a current level of Iw2. Then, each current level ofIw0, Iw1 and Iw2 is optimally determined for predetermined power levels,respectively. When information is written on the disc in a write mode,first, a w0 D/A converter is supplied with data for the current valueIw0, a w1 D/A converter is supplied with data for the current value Iw1and a w2 D/A converter is supplied with data for the current value Iw2.Then, switches SW4, SW5 and SW6 are turned on or turned off according tothe data to be written. As a result, the laser diode LD1 is suppliedwith the LD drive current.

The Japanese Laid-Open Patent Application No. 11-25456 (a prior artmethod {circle around (2)}) also discloses such kind of technology. Theprior art method {circle around (2)} discloses a simple method forrecording multi level information on an optical information medium inorder to raise recording density. In the prior art method {circle around(2)}, initially, first pits are formed on a re-writable media (a phasechange optical disc) by radiating a laser light. Then, second pits areformed on the first pits by erasing parts of the first pits according toa multi leveled signal of the information to be recorded. Sizes of thesecond pits are modulated according to the multi level information.

FIG. 1 shows an example of a waveform of a recording pulse signalaccording to the prior art method {circle around (1)} as describedabove. The waveform of the recording pulse signal is composed of arectangular recording pulse and an erasing pulse. A means for adjustingthe size of a recorded mark controls the width of the recording pulse(T3−T1 as shown in FIG. 1) or strength of a light (Pw as shown in FIG.1). On reproducing the recorded marks, strength of a reflected light bythe mark is varied according to the size of the recorded mark.Therefore, a multi level signal, which is converted from the lightreflected and has one of MARK signal levels corresponding to the size ofthe recorded mark, can be obtained as shown in FIG. 2. FIG. 2 shows aprinciple of such multi level recording method using a recorded marksize modulation.

A disadvantage of this method using the waveform of the recording pulsesignal composed of the rectangular recording pulse and the erasing pulseis that the recorded mark is apt to be enlarged in the radial directionbecause of thermal diffusion caused by the laser beam focused on theoptical information medium. Therefore, the shape of the recorded mark isapt to have a shape like a drop of water as shown in FIG. 1. Therefore,this method can not control the influence of the thermal diffusion onthe size of the recorded mark if the size of the mark smaller than thesize of the focused laser beam spot is formed.

FIG. 3 shows a relation between an occupied mark ratio and a multi levelmark signal level according to the prior art method {circle around (1)}.As shown in FIG. 3, at a region {circle around (1)} in which the marksignal level is relatively high, a ratio of the mark signal levelvariation to the occupied mark ratio variation is high compared to thatat a region {circle around (2)} in which the mark signal level isrelatively low. Therefore, there is a need to control the occupied markratio more precisely at the region {circle around (1)} than at theregion {circle around (2)}. However, the prior art method {circle around(1)} as shown in FIG. 1 cannot control the occupied mark ratioaccurately because it cannot control the influence of the thermaldiffusion to the size of the recorded mark. Therefore, the prior artmethod {circle around (1)} cannot control each level of the multi levelsignal accurately.

FIG. 5 shows the prior art method {circle around (2)}. To eliminate thedisadvantage of the prior art method {circle around (1)}, it is possibleto use a recording pulse signal composed of multi pulses as shown inFIG. 5. In this prior art method {circle around (2)}, the recordingpulse is composed of a start pulse, short multi pulses and an erasingpulse. As the short multi pulses are used, the enlargement of therecorded mark in the radial direction caused by thermal diffusion can beprevented. A means for adjusting the size of the recorded mark controlsthe width of the first pulse (T2−T1 as shown in FIG. 5), the duty of themulti pulses (T4−T3 as shown in FIG. 5), or strength of the first pulseand the multi pulses (Pw as shown in FIG. 5).

There is a need to raise the frequency of the recording pulses to formthe recorded marks having a size less than that of the focused laserbeam spot. Therefore, a recording pulse generation circuit and a laserdiode need to be operated at a high frequency in order to record theinformation on the optical information medium at a high speed. However,it is hard to raise their operating frequencies. Further, the recordingpulse generation circuit becomes complex.

SUMMARY OF THE INVENTION

It is a general object of the present invention to provide a method foroptically recording information in which the above disadvantages areeliminated.

Further, it is another object of the present invention to provide amethod for optically recording information in which recorded markshaving a size less than that of a focused laser beam spot can be formedusing a multi level signal recording method with a recording pulsehaving a simple waveform.

Further, it is another object of the present invention to provide amethod for optically recording information in which the size of eachrecorded mark having a size less than that of a focused laser beam spotcan be controlled to a desired size.

Further, it is another object of the present invention to provide amethod for optically recording information in which a position of therecorded mark having a size less than that of a focused laser beam spotis controlled precisely by a simple method for controlling the recordingpulse.

Further, it is another object of the present invention to provide amethod for optically recording information in which a size of therecorded mark having the size less than that of a focused laser beamspot is fine-tuned accurately.

Further, it is another object of the present invention to provide amethod for optically recording information in which the position of therecorded mark having a size less than that of a focused laser beam spotis fine-tuned accurately.

Further, it is another object of the present invention to provide amethod for optically recording information in which each level of themulti level signal is recorded accurately.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium having at least one track, in which method a laser light isfocused to a laser spot to form a mark on the optically re-writableinformation medium and the size of the mark is modulated according to alevel of a multi-level signal which correspond to the information to berecorded, further in which method:

the strength of the laser light for recording each mark is modulatedaccording to a waveform comprising a first rectangular erasing pulsesignal, a rectangular recording pulse signal, an off-pulse signal and asecond rectangular erasing pulse signal,

the laser light having a strength set to a value indicated by the firstrectangular erasing pulse signal can erase a recorded mark,

the laser light having a strength set to a value indicated by therectangular recording pulse signal can record a mark,

the strength of the laser light set to a value indicated by theoff-pulse signal is less than the strength of the laser light used inreproduction of the recorded marks,

the laser light having a strength set to a value indicated by the secondrectangular erasing pulse signal can erase the recorded mark, and

a product of the time interval of the rectangular recording pulse signaland the relative linear velocity between the laser spot and theoptically re-writable information medium is shorter than the length ofthe recorded mark.

According to the present invention, the mark is formed on a recordingmedium by radiating the laser light having the strength of the shortrecording pulse signal followed by the off-pulse signal. The recordingfilm is heated to form the mark and the mark is extended by a thermaldiffusion while the off-pulse signal is provided. Therefore, the lengthof the recorded mark ML is longer than the recording pulse width at thepredetermined linear velocity. The erasing pulse signal follows theoff-pulse signal and the extended recorded mark is erased by the erasingpulse signal.

The recording method according to the present invention can form a markon the recording medium by radiating the laser light having a shorterrecording pulse width than a recording pulse width used in theconventional method to form a mark having the same length. Therefore,heat quantity applied to the medium by the recording method according tothe present invention is smaller than that of the conventional method.As a result, expansion of the mark in a direction perpendicular to thelongitudinal direction of the track is reduced compared to theconventional methods.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the size of the mark is modulated according to theinformation by means of modulating a pulse width of the off-pulse signalso that the time interval between a rising edge of the rectangularrecording pulse signal and a rising edge of the rectangular erasingpulse signal corresponds to the size of the mark.

According to the present invention, a simple recording pulse signalgeneration circuit can be used for multi-level recording because thesize of the mark is modulated according to the information by means ofmodulating only the pulse width of the off-pulse signal.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the track is divided into a plurality of cells and themark is recorded in the cell so that the center of the mark is placed atthe center of the cell by means of adjusting both the rising edge of therectangular recording pulse signal and the rising edge of therectangular erasing pulse signal.

According to the invention, the center of the mark is precisely placedat the center of the cell by means of adjusting both the rising edge ofthe rectangular recording pulse signal and the rising edge of therectangular erasing pulse signal because the effect of the thermaldiffusion is effectively used.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the size of the mark is fine-adjusted by controlling thetiming of a falling edge of the rectangular recording pulse signal.

According to the invention, the size of the mark is fine-adjusted bycontrolling only the timing of the falling edge of the rectangularrecording pulse signal because the effect of the thermal diffusion iseffectively used.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the size of the mark is fine-adjusted by controlling thestrength of the laser light modulated by the rectangular recording pulsesignal.

According to the present invention, the size of the mark isfine-adjusted by controlling only the strength of the laser lightmodulated by the rectangular recording pulse signal because the effectof the thermal diffusion is effectively used.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the timing of the rising edge of the rectangularrecording pulse signal is controlled so that the center of the mark isplaced at the center of the cell.

According to the invention, the center of the mark is precisely placedat the center of the cell by only further shifting the rectangularrecording pulse signal because the effect of the thermal diffusion iseffectively used.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the relation between the length (ML) of the mark alongthe track and a diameter (BD) of the laser spot satisfies ML≦BD, and

the product of the pulse width of the rectangular recording pulse signaland the relative linear velocity is less than 20% of the diameter (BD)of the laser spot.

According to the present invention, the width of the rectangularrecording pulse signal is properly limited so that the mark can berecorded with higher precision.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein the relation between the length of a minimum mark(MLmin) along the track, except when there is no mark, and the diameter(BD) of the laser spot satisfies MLmin/BD≧0.10, and

each level of the multi-level signal is assigned so that the leveldifference between adjacent levels becomes equidistant, except whenthere is no mark.

According to the present invention, the deviation from the aimedmulti-level signal levels can be reduced.

The above objects of the present invention are achieved by a method foroptically recording information on an optically re-writable informationmedium, wherein

the relation between the length of a maximum mark (MLmax) along thetrack, except for a mark that has about the same length as a diameter ofthe laser spot, and the diameter (BD) of the laser spot satisfiesMLmax/BD≦0.70, and

each occupied mark ratio of the multi-level signal is assigned so thatthe difference between the occupied mark ratios corresponding toadjacent levels of the multi-level signal becomes equidistant, exceptfor the mark that has about the same length as the diameter of the laserspot.

According to the present invention, the deviation from the aimedmulti-level signal levels can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed description when readin conjunction with the accompanying drawings, in which:

FIG. 1 shows an example of a waveform of a recording pulse signalaccording to the prior art method {circle around (1)};

FIG. 2 shows a principle of a multi level recording method using arecorded mark size modulation according to the prior art;

FIG. 3 shows a relation between an occupied MARK ratio and a MARK signallevel according to the prior art method {circle around (1)};

FIG. 4 shows measured deviation values from aimed multi-level signallevels according to the prior art method {circle around (1)};

FIG. 5 shows an example of a waveform of a recording pulse signalaccording to the prior art method {circle around (2)};

FIG. 6 shows an example of a basic waveform of a recording pulse signalaccording to an embodiment of the present invention;

FIG. 7 shows a waveform of a recording pulse signal of a first methodfor controlling the width of a recorded mark in a directionperpendicular to a longitudinal direction of a track according to anembodiment of the present invention;

FIG. 8 shows a waveform of a recording pulse signal of a second methodfor controlling the width of a recorded mark in a directionperpendicular to the longitudinal direction of the track according to anembodiment of the present invention;

FIG. 9 shows a waveform of a recording pulse signal of a third methodfor controlling the width of a recorded mark in a directionperpendicular to the longitudinal direction of the track according to anembodiment of the present invention;

FIG. 10 shows a waveform of a recording pulse signal of a fourth methodfor controlling the width of a recorded mark in a directionperpendicular to the longitudinal direction of the track according to anembodiment of the present invention;

FIG. 11 shows a waveform of a recording pulse signal of a first methodfor controlling the start point of a recorded mark according to anembodiment of the present invention;

FIG. 12 shows a waveform of a recording pulse signal of a second methodfor controlling the start point of a recorded mark according to anembodiment of the present invention;

FIG. 13 shows a waveform of a recording pulse signal of a third methodfor controlling the start point of a recorded mark according to anembodiment of the present invention;

FIG. 14 shows a waveform of a recording pulse signal of a fourth methodfor controlling the start point of a recorded mark according to anembodiment of the present invention;

FIG. 15 shows relations between waveforms of recording pulse signals andrecorded marks according to an embodiment of the present invention;

FIG. 16 shows a block diagram of an optical information recordingapparatus according to an embodiment of the present invention;

FIG. 17 shows a result of recording and reproducing information on adisc according to an embodiment of the present invention;

FIG. 18 shows measured deviation values from aimed multi-level signallevels according to an embodiment of the present invention;

FIG. 19 shows a relation between a MARK signal level of a multi level RFsignal and an occupied MARK ratio;

FIG. 20 shows a relation between a MARK signal level of a multi level RFsignal and an occupied MARK ratio;

FIG. 21 shows a relation between an occupied MARK ratio and a MARKsignal level of a multi level RF signal;

FIG. 22 shows a relation between an occupied MARK ratio and a MARKsignal level of a multi level RF signal;

FIG. 23 shows relation between the mark levels and the occupied markratios (ML/BD);

FIG. 24 shows the relation between the mark levels and the occupied markratios;

FIG. 25 shows the relation between the occupied mark ratios and the marklevels; and

FIG. 26 shows the relation between the occupied mark ratio and the marklevel difference ΔS.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, an embodiment according to the present invention will be explainedwith reference to accompanying figures.

1. Principle of Recording

FIG. 6 shows a waveform of a recording pulse signal according to thepresent invention. The waveform of the recording pulse signal, by meansof which strength of a laser light supplied from a laser diode ismodulated, consists of a first rectangular erasing pulse signal, arectangular recording pulse signal, an off-pulse signal and a secondrectangular erasing pulse signal, and these four kinds of pulse signalsare repeated for each mark. The laser light having the strength that isset to a value indicated by the first rectangular erasing pulse signalcan erase a recorded mark. The laser light having the strength that isset to a value indicated by the rectangular recording pulse signal canform a mark. The strength of the laser light used in reproduction of therecorded marks is set to a value less than the value of the rectangularrecording pulse by means of the off-pulse signal. The laser light havingthe strength which is set to a value indicated by the second rectangularerasing pulse signal can erase a recorded mark. A track is divided intocells in a longitudinal direction. Each cell corresponds to a timeinterval T5 at a predetermined linear velocity as shown in FIG. 6.Length of a mark ML recorded in a cell along the track can be controlledby recording pulse width (T2−T1) and a start point of the erasing pulsesignal T4. If the length of the mark to be recorded is changed, a timeT1 and the time T4 are adjusted to record the mark at the center of thecell. For example, if the length of the recorded mark is changed from MLto ML′, then T1 should also be changed to T1+s and T4 should also bechanged to T4−s to record the mark at the center of the cell.

The mark ML is formed on a recording medium by radiating the laser lighthaving the strength of the recording pulse signal during the timeinterval T2−T1. The recording film is heated to form the mark and themark is extended by thermal diffusion while the off-pulse signal isprovided. Therefore, the length of the recorded mark ML is longer thanthe recording pulse width at the predetermined linear velocity. Theerasing pulse signal follows the off-pulse signal and the extendedrecorded mark is erased by the erasing pulse signal. As a result, themarks ML as shown in FIG. 6 are formed on the recording medium.

The recording method according to the present invention can form a markon the recording medium by radiating the laser light having a shorterrecording pulse width (T2−T1) than the recording pulse width used in theconventional method to form a mark having the same length. Therefore,heat quantity applied to the medium by the recording method according tothe present invention is smaller than that of the conventional method.As a result, expansion of the mark in a direction perpendicular to thelongitudinal direction of the track is reduced compared to theconventional methods. The present invention has such an advantage.Further, the size of the recorded mark can be controlled by means ofchanging the off-pulse width according to each level of a desiredmulti-level signal based on the principle of the present invention.

2. Controlling the Mark Width in a Direction Perpendicular to theLongitudinal Direction of the Track

As described above, by limiting the effect of the thermal diffusion, itis possible to control the mark width in a direction perpendicular tothe longitudinal direction of the track. The thermal quantity applied bythe laser light radiated to the medium is controlled to limit theextension of the mark.

FIG. 7 through FIG. 10 show a method to control the mark width in adirection perpendicular to the longitudinal direction of the track. Asshown in FIG. 7, the mark width is extended as the recording pulse widthis extended from the time interval T2 to the time interval T21, whereT21 is greater than T2. The mark A is recorded with a recording pulse T2and the mark B is recorded with a recording pulse T21.

On the contrary, as shown in FIG. 9, the mark width is reduced as therecording pulse width is reduced from the time interval T2 to the timeinterval T22, where T22 is shorter than T2. The mark A is recorded witha recording pulse T2 and the mark B is recorded with a recording pulseT22.

As shown in FIG. 8, the mark width is extended as the strength of thelaser light is raised from PW1 to PW2, where PW2 is greater than PW1.The mark A is recorded with the laser light having the strength of PW1and the mark B is recorded with the laser light having the strength ofPW2.

On the contrary, as shown in FIG. 10, the mark width is reduced as thestrength of the laser light is reduced from PW1 to PW3, where PW3 islower than PW1. The mark A is recorded with the laser light having thestrength of PW1 and the mark B is recorded with the laser light havingthe strength of PW3.

3. Controlling the Position of the Recorded Mark

As described above, the thermal quantity applied by the laser lightradiated to the medium is controlled to limit the extension of the markin the direction perpendicular to the longitudinal direction of thetrack. As a result, the start point of the recording mark formed on themedium is also changed. Therefore, the relative position between therecorded mark and the center of the cell is changed.

Therefore, the mark position recorded along the track also needs to becontrolled when the mark width is controlled. FIG. 11 through FIG. 14shows several methods to control the start position for recording themark.

As shown in FIG. 11, the start point of the recording pulse signal ismodified from T1 to T1−s and the end point of the recording pulse signalis modified from T21 to T21+s to correct a forward variation s of thestart point without changing the pulse width T21−T1.

As shown in FIG. 13, the start point of the recording pulse signal ismodified from T1 to T1−s and the end point of the recording pulse signalis modified from T22 to T22−s to correct a backward variation s of thestart point without changing the pulse width T22−T1.

As shown in FIG. 12, the start point of the recording pulse signal ismodified from T1 to T1+s and the end point of the recording pulse signalis modified from T2 to T2+s to correct a forward variation s of thestart point without changing the pulse width T2−T1.

As shown in FIG. 14, the start point of the recording pulse signal ismodified from T1 to T1−s and the end point of the recording pulse signalis modified from T2 to T2−s to correct a backward variation s of thestart point without changing the pulse width T2−T1.

FIG. 15 shows relations between waveforms of recording pulse signals andrecorded marks according to an embodiment of the present invention.

4. Embodiment {circle around (1)} of Recording and Reproduction

FIG. 16 shows a block diagram of an optical information recordingapparatus according to an embodiment of the present invention. In thisembodiment {circle around (1)}, the information is recorded andreproduced on an optical recording medium 1 by the optical informationrecording apparatus. The optical information recording apparatus asshown in FIG. 16 has a spindle motor (not shown), a laser diode 2, acollimator lens 3, a polarizer 4, an objective lens 5, a detection lens6, a quadrant photo detector 7 and actuators 8. The light radiated fromthe laser diode 2 is focused on the optical recording medium 1 throughthe collimator lens 3, the polarizer 4 and the objective lens 5, andthen, the marks are recorded on the optical recording medium 1. Thereflected light is polarized by the polarizer 4 and focused on thequadrant photo detector 7 through the detection lens 6. Then, an RFsignal is detected by the quadrant photo detector 7. The actuators 8control the position of the objective lens 5 both for focusing the laserspot on the medium and for moving the laser spot to follow the track. Awavelength λ of the laser for recording or reproduction is 650 nm and anumerical aperture NA is 0.65. As a result, the diameter of the laserbeam spot is 0.8 μm. These parameters are commonly used in a typicalre-writable DVD recording apparatus. The optical recording medium 1 iscomposed of a phase change material such as AgInSbTe, which is are-writable type material. The marks are recorded on a groove track anda distance between the adjacent tracks, which is called a track pitch,is 0.74 μm and the track width is about 0.4 μm. Recording conditions areas follows. The recording power is 14 mW on the recording surface, theerasing power is 7.5 mW and the relative linear velocity between thegroove track and the laser light spot is about 3.5 m/s. The length ofthe cell is about 0.7 μm along the groove track.

FIG. 17 shows a result of recording and reproducing information on theoptical recording media 1 according to an embodiment of the presentinvention. In FIG. 17, the abscissa shows a ratio of a recording pulsewidth (T2−T1) to a beam spot diameter BD and the ordinate shows a sumsignal that is called the RF signal. The RF signal is generated by meansof opto-electrically converting a light reflected from the recorded markto an electrical signal. The curve Lv1 in FIG. 17 shows a case when therising edge T1 of the recording pulse signal and the falling edge T2 ofthe erasing pulse signal are adjusted so that the mark length of therecorded mark ML becomes 0.13 μm. The curve Lv2 in FIG. 17 shows a casewhen the rising edge T1 of the recording pulse signal and the fallingedge T2 of the erasing pulse signal are adjusted so that the mark lengthof the recorded mark ML becomes 0.27 μm. The curve Lv3 in FIG. 17 showsa case when the rising edge T1 of the recording pulse signal and a thefalling edge T2 of the erasing pulse signal are adjusted so that themark length of the recorded mark ML becomes 0.40 μm.

FIG. 17 shows that the RF signal value (Mark signal level) for eachcurve is saturated at a range of the recording pulse width ratio(T2−T1)/BD between 0.16 to 0.20. This is because the maximum mark widthin a direction perpendicular to the longitudinal direction of the trackis limited to the groove track width.

These data mean that the RF signal level (Mark signal level) can befine-adjusted as long as the recording pulse signal ratio of (T2−T1)/BDis kept less than 0.20.

5. Embodiment {circle around (2)} of Recording and Reproduction

This embodiment {circle around (2)} shows deviation values from aimedmulti-level signal levels.

FIG. 18 shows measured deviation values from aimed multi-level signallevels according to an embodiment {circle around (2)} of the presentinvention. On the other hand, FIG. 4 shows measured deviation valuesfrom aimed multi-level signal levels according to the prior art method{circle around (1)}. The recording condition used in this embodiment{circle around (2)} is the same condition as that used in the embodiment{circle around (1)}.

As described for the prior art methods, at the region {circle around(1)} as shown in FIG. 3 in which a mark signal level is relatively high,the mark signal level variation to the occupied mark ratio variation ishigh compared to that at the region {circle around (2)} in which a marksignal level is relatively low. In the prior art methods, the maximumdeviation (σ) to the aimed multi-level signal level is about 8%.

On the contrary, the result from this embodiment {circle around (2)}shows that the maximum deviation (σ) to the aimed multi-level signallevel at the region {circle around (1)} is about 2%. This means that themethod according to the present invention is superior to the prior artmethods.

6. Embodiment {circle around (3)} for Determining Levels of aMulti-Level Signal

In the multi-level signal recording method in which a size of therecorded mark is changed according to the signal level, it is importantto select a condition of the mark length, under which the mark is easilyrecorded in order to easily detect the multi-level RF signal correctly.FIG. 19 and FIG. 20 show the relation between the multi-level RF signallevel (Mark signal level) and the occupied mark ratio. The abscissashows the multi-level RF signal level (Mark signal level)and theordinate shows the occupied mark ratio.

As shown in FIG. 19, a range of the mark signal level between 0.4 and1.0 can be used to express the multi-level signal. The black dots show 8multi-levels. The distances between the adjacent levels are equal toeach other. FIG. 23 shows the relation between the mark levels and theoccupied mark ratios (ML/BD) shown in FIG. 19.

As shown in FIG. 23, the occupied mark ratio difference ΔL is 0.04between the mark levels of 1.00 and 0.91. Therefore, there is littlemargin in the precision for forming the mark ML. On the other hand, theoccupied mark ratio difference ΔL is more than 0.1 between the marklevels of 0.40 and 0.66. Therefore, there is much margin in theprecision for forming the mark ML.

An embodiment {circle around (3)} is shown in FIG. 24. FIG. 24 shows therelation between the mark levels and the occupied mark ratios, in whichthe margin in the precision for forming the mark is raised for highermark levels. In the FIG. 24, the occupied mark ratio difference ΔL israised to about twice the occupied mark ratio difference ΔL as shown inFIG. 23 for higher mark levels. The minimum occupied mark ratio for themark level of 0.80 is set to 0.12 in FIG. 24. The mark level of 1.00 isruled out because there is no mark for the mark level of 1.00. As aresult, 7 mark levels are equidistantly assigned between the mark levelof 0.40 and 0.80 in FIG. 24. Therefore, the minimum occupied mark ratiofor the cell, except for the cell without mark having the mark level of1.00, can be set more than 0.1 so that the margin of ΔL is leveled andthe deviation from the aimed multi-level signal can be reduced.

7. Embodiment {circle around (4)} for Determining Levels of aMulti-Level Signal

As mentioned above, in the embodiment {circle around (3)}, the methodfor enhancing the margin of ΔL, in which method the minimum occupiedmark ratio for the mark level can be set more than 0.1 so that themargin of ΔL is leveled, is described. In this embodiment {circle around(4)}, a method for reducing the deviation of the multi-level signal fromthe aimed multi-level signal while the margin of ΔL is leveled, isdescribed. Both FIG. 21 and FIG. 22 show the relation between theoccupied mark ratio (ML/BD) and the mark signal level. An abscissa showsthe occupied mark ratio (ML/BD) and an ordinate shows the mark signallevel.

As shown in FIG. 21, each mark level of 8 mark levels is assigned sothat the occupied mark ratio is between 0 to 1.0 and ΔL is equidistantlyassigned. FIG. 25 shows the relation between the occupied mark ratiosand the mark levels under the condition mentioned above in FIG. 21.

As shown in FIG. 25, the mark level difference ΔS is 0.01 between theoccupied mark ratios of 0.71 and 1.00. There is little margin in theprecision for forming the mark ML.

An embodiment {circle around (4)} is shown in FIG. 26. FIG. 26 shows therelation between the occupied mark ratio and the mark level differenceΔS, in which the margin of ΔS is raised compared to FIG. 25.

As shown in FIG. 26, the maximum occupied mark ratio is set to 0.70. Themark level of 1.00 is ruled out because the cell is filled with the markfor the mark level of 1.00. As a result, 7 mark levels are equidistantlyassigned between the occupied mark ratios of 0 and 0.70. Therefore, ΔSis leveled and the minimum value of ΔS is improved to 0.05.

As described above, the maximum occupied mark ratio can be set to 0.70so that the margin of ΔS is leveled and the deviation of the multi-levelsignal from the aimed multi-level signal can be reduced.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

The present application is based on Japanese priority applicationNo.2001-036395 filed on Feb. 14, 2001, the entire contents of which arehereby incorporated by reference.

1. An optical information recording apparatus comprising: means forfocusing a laser light to a laser spot to form a mark on an opticalrecording medium; and means for modulating a size of said mark accordingto a level of a multi-level signal that corresponds to information to berecorded, and wherein a strength of said laser light for recording eachmark is modulated according to a waveform comprising a first rectangularerasing pulse signal, a rectangular recording pulse signal, an off-pulsesignal and a second rectangular erasing pulse signal, said laser lighthaving a strength set to a value indicated by said first rectangularerasing pulse signal can erase a recorded mark, said laser light havinga strength set to a value indicated by said rectangular recording pulsesignal can record a mark, a strength of said laser light set to a valueindicated by said off-pulse signal is less than a strength of said laserlight used in reproduction of said recorded marks, said laser lighthaving a strength set to a value indicated by said second rectangularerasing pulse signal can erase said recorded mark, and a product of thetime interval of said rectangular recording pulse signal and a relativelinear velocity between said laser spot and said medium is shorter thana length of said recorded mark, and wherein a length of said rectangularrecording pulse signal is set to a predetermined constant value and alength of said off-pulse signal is adjusted according to saidmulti-level signal.
 2. The apparatus of claim 1, wherein a size of saidmark is modulated according to said information by means of modulating apulse width of said off-pulse signal so that a time interval between arising edge of said rectangular recording pulse signal and a rising edgeof said rectangular erasing pulse signal corresponds to said size ofsaid mark.
 3. The apparatus of claim 1, wherein said apparatus isarranged such that a center of said mark is placed at a center of a cellby means of adjusting both a rising edge of said rectangular recordingpulse signal and a rising edge of said rectangular erasing pulse signal.4. The apparatus of claim 2, wherein said apparatus is arranged suchthat a center of said mark is placed at a center of said cell by meansof adjusting both said rising edge of said rectangular recording pulsesignal and said rising edge of said rectangular erasing pulse signal. 5.The apparatus of claim 1, further comprising means for controllingtiming of a falling edge of said rectangular recording pulse signal. 6.The apparatus of claim 2, wherein said size of said mark isfine-adjusted by controlling timing of a falling edge of saidrectangular recording pulse signal.
 7. The apparatus of claim 3, whereinsaid apparatus is arranged such that said size of said mark isfine-adjusted by controlling timing of a falling edge of saidrectangular recording pulse signal.
 8. The apparatus of claim 4, whereinsaid size of said mark is fine-adjusted by controlling timing of afalling edge of said rectangular recording pulse signal.
 9. Theapparatus of claim 1, further comprising means for controlling saidstrength of said laser light.
 10. The apparatus of claim 2, wherein saidsize of said mark is fine-adjusted by controlling said strength of saidlaser light.
 11. The apparatus of claim 3, wherein said apparatus isarranged such that the size of said mark is fine-adjusted by controllingsaid strength of said laser light.
 12. The apparatus of claim 4, whereinsaid size of said mark is fine-adjusted by controlling said strength ofsaid laser light.
 13. The apparatus of claim 5, wherein said apparatusis arranged such timing of a rising edge of said rectangular recordingpulse signal is controlled so that a center of said mark is placed at acenter of a cell.
 14. An apparatus for recording information on amedium, said apparatus comprising: a source of laser light; and amodulator for modulating a size of a mark according to a level of amulti-level signal that corresponds to information to be recorded, andwherein a strength of said laser light is modulated according to awaveform comprising a first rectangular erasing pulse signal, arectangular recording pulse signal, an off-pulse signal and a secondrectangular erasing pulse signal, said laser light having a strength setto a value indicated by said first rectangular erasing pulse signal canerase a recorded mark, said laser light having a strength set to a valueindicated by said rectangular recording pulse signal can record a mark,a strength of said laser light set to a value indicated by saidoff-pulse signal is less than a strength of said laser light used inreproduction of said recorded marks, said laser light having a strengthset to a value indicated by said second rectangular erasing pulse signalcan erase said recorded mark, and a product of the time interval of saidrectangular recording pulse signal and a relative linear velocitybetween a laser spot and said medium is shorter than a length of saidrecorded mark.
 15. The apparatus of claim 14, wherein said apparatus isarranged such that a relation between the length (ML) of said mark alonga track and a diameter (BD) of said laser spot satisfies ML≦BD, and aproduct of the pulse width of said rectangular recording pulse signaland said relative linear velocity is less than 20% of said diameter (BD)of said laser spot.
 16. The apparatus of claim 14, wherein a relationbetween a length of a minimum mark (MLmin) along said track except whenthere is no mark and a diameter (BD) of said laser spot satisfiesMLmin/BD≧0.10, and each level of said multi-level signal is assigned sothat the level difference between adjacent levels becomes equidistantexcept when there is no mark.
 17. The apparatus of claim 14, whereinsaid apparatus is arranged such that a relation between a length of amaximum mark (MLmax) along said track, except for a mark that has aboutthe same length as a diameter of said laser spot, and a diameter (BD) ofsaid laser spot satisfies MLmax/BD≦0.70, and each occupied mark ratio ofsaid multi-level signal is assigned so that the difference between saidoccupied mark ratios corresponding to adjacent levels of saidmulti-level signal becomes equidistant except for said mark that hasabout the same length as said diameter of said laser spot.